In the semiconductor industry, it is very common to process a disc-like substrate in a vacuum chamber. Often, there is a requirement to heat or cool the substrate while ensuring that a whole face of the substrate is processed. The latter requirement eliminates the possibility of clamping the substrate mechanically at its periphery and the principle of clamping the wafer electrostatically to a heat source or heat sink is well established. The basic principle involves the use of a power supply to apply a voltage to one or more electrodes embedded inside a dielectric support that in turn contacts the substrate(s) to be processed. The applied voltage creates a field that induces a charge in the surface of the substrate in contact with the dielectric support that is opposite in polarity to the charge on the electrode. The opposite charges attract each other by electrostatic attraction and thus the substrate is clamped to the support. When the charge on the electrode is removed, the electric field that it created relaxes and the charge induced on the substrate dissipates. The substrate will then detach from the support.
Though simple in concept, in practice there are many difficulties encountered with the use of electrostatic clamps or chucks (often referred to as ESCs). A large number of different embodiments have attempted to address many of these practical difficulties. One problem is encountered when the substrate is warped. The clamping force is proportional to the square of the distance between the ESC electrode and the substrate and also to the area of the substrate that is attracted: consequently, the clamping force on a warped substrate will be lower than that on a completely planar substrate. When there is a need to process warped substrates, ESCs with high clamping forces are required. High clamping forces can be generated by applying high voltages to the ESC electrodes or by the use of the Johnson-Rahbek effect. Johnson-Rahbek or J-R ESCs incorporate a leaky dielectric (with a resistivity typically in the range of 10E8 Ω-cm to 10E13 Ω-cm) to generate charge at the surface of the ESC in direct contact with the substrate. J-R ESCs generate higher forces than regular “coulombic” ESCs (with resistivities >10E14 Ω-cm) for similar applied voltages. However, the use of J-R ESCs or coulombic ESCs at high applied voltages to generate high clamping forces often makes substrate release more difficult.
The thermal coupling between ESC and substrate is often enhanced by the use of gas between them. Typically, an inert gas such as helium or argon is preferred at pressures in the range 1 Torr to 20 Torr. In order to achieve these pressures with low flows of gas, a seal ring at the periphery of the ESC is required: this seal ring usually takes the form of a raised ridge. Additional features are incorporated into the surface of the ESC to ensure uniform distribution of this gas across the whole backside surface of the substrate.
One common surface feature detail is the so-called “MCA” or “minimum contact area” ESC. Here, the surface of the ESC comprises a plurality of protrusions that are raised above the rest of the chuck surface. The area of the raised protrusions that contact the substrate is only a very small fraction (typically ˜1%) of the total ESC area. There are several advantages to the MCA ESC. When a substrate is clamped to an ESC, the electrostatic attraction usually flattens the substrate, causing relative motion between the two bodies. This sliding motion can generate particles that adhere either to the back of the substrate or to the surface of the ESC. Both outcomes are undesirable in semiconductor manufacturing: particles on the backside of a substrate can fall onto the front side of the substrate below it in the cassette, reducing device yield, while particles on the surface of the ESC can cause clamping difficulties or gas leakage. An MCA ESC reduces the contact area between chuck and substrate, reducing the probability that particles will be generated. In the event that small particles are generated, they are most likely to fall into the interstices between protrusions where they do not impair ESC operation. The MCA ESC also is less likely to experience declamping problems because of the limited contact area between chuck and substrate.
Prior Art
The most common substrates in use for semiconductor manufacturing today are silicon (Si) and gallium arsenide (GaAs) wafers, though numerous other substrates such as SiC, sapphire and quartz are used in emerging technologies (e.g., LED manufacturing). In the case of silicon, many of the wafers processed have a dielectric on the back surface that contacts the chuck. Both Si and GaAs substrates are readily electrostatically clamped by the many ESCs available on the market today. However, often substrates (particularly sapphire, semi-insulating GaAs and Si with dielectric coatings on the backside) do not release quickly and cleanly from the ESC when the applied voltage is removed. In the prior art, there are three main techniques that address this problem.
The first and simplest technique uses a mechanical force to push the substrate off the ESC. This force can take the form of a plurality of pins arranged over the surface of the ESC that are actuated when declamping is required. Each pin exerts a small force on part of the substrate and their combined force overcomes any residual electrostatic attraction holding the substrate to the ESC. Gas pressure can also be applied to the back of the substrate, in addition to or without pins, to overcome the residual clamping force.
A second method uses a time varying potential applied to the ESC electrodes to dissipate the residual charge on the surface of the ESC and the back of the substrate. In one form, an evanescent sine wave is applied to the output of the ESC power supply. In another, the potential applied to the ESC is perturbed and a signal (usually a capacitance) that correlates to the residual clamping force on the wafer is monitored. The applied potential is varied until the value corresponding to the minimum residual clamping force is determined, at which time the declamping operation is commenced, usually by lifting the substrate off the ESC with pins.
A third method seeks to prevent the build up of significant charge at the interface between ESC and substrate by continuously varying the applied voltage during clamping. Often, this is achieved by the use of a multipolar chuck that consists of three or more electrodes. The electrodes are driven either by alternating current (AC) or by square wave output. At all times, there is an applied potential to a least one electrode, so the substrate experiences some clamping force. However, because the sign of the applied potential is periodically reversed, the charge carriers are constantly in motion and a large residual charge does not develop on any part of the ESC or substrate. Variants of this technique are also possible with monopolar or bipolar ESCs.
The patent literature contains references where ESCs are fabricated in such a way that metal or metal coatings contact the substrate during electrostatic clamping. For example, in U.S. Pat. No. 4,502,094, Lewin et al. describe an ESC where metal posts and an external metal tubular support contact the substrate. The purpose of said metal components is to improve the thermal contact between the wafer and the ESC and to act as a heat sink.
In U.S. Pat. No. 5,745,332, Burkhart et al. describe the use of a partial metal coating or perforated screen to provide a second electrical contact to a monopolar ESC, thereby enabling electrostatic clamping without the need to ignite a plasma.
In U.S. Pat. No. 7,154,731, Kueper describes the use of a partially metalized ESC surface to decrease radiative coupling between ESC and substrate. Here, the inventor teaches that the metallic regions of the ESC should not contact the substrate.
Disadvantages of the Prior Art
There are two main disadvantages to the use of mechanical force to remove a substrate from an ESC. If the substrate is firmly attached to the ESC and too much force is applied, the substrate is easily broken, either by pins pushing up through the substrate or if the substrate jumps off the ESC, falls and breaks on landing. Also, failing to remove the residual charge on both the ESC and wafer can create problems. In the case of the ESC, the residual charge can impair clamping or declamping of the next wafer processed. A substrate with residual charge on its surface is subject to mishandling in subsequent processing.
Use of a dechucking operation that applies a time varying potential to the ESC electrodes is a very effective technique if the substrate and processes remain constant. However, often, the type of substrate or the process recipe will change. This frequently results in the need to develop a new dechucking algorithm. Sticking during declamping can be a relatively infrequent, sporadic occurrence, and the need to process many hundreds or thousands of substrates through the equipment to determine that a new dechucking algorithm is effective can be a costly and time consuming process. In addition, sometimes a dechucking algorithm that has worked for a long time over thousands of wafers can start to fail. Usually, the cause is difficult (if not impossible) to ascertain and the only solution is to develop a new algorithm empirically.
Preventing build up of too much residual charge during processing is a very elegant solution, but the need to use a complicated and expensive multipolar ESC and power supply is a significant disadvantage. Furthermore, some implementations of the multipolar ESC approach cause the wafer to vibrate at the frequency of the applied square wave signal as each pole clamps and then releases the substrate. This vibration can generate large numbers of particles on the back side of the substrate or can damage and break thin substrates.
There are no obvious disadvantages to the inventions described in U.S. Pat. No. 5,745,332 and U.S. Pat. No. 7,154,731, but the inventors are addressing distinctly different problems from the sticking problem addressed herein. U.S. Pat. No. 5,745,332 limits its claims to monopolar ESCs while U.S. Pat. No. 7,154,731 teaches that the metal coating should be applied to regions that do not contact the substrate. In addition, in one preferred embodiment, U.S. Pat. No. 7,154,731 describes a method to protect the metal surfaces by an additional dielectric covering. This method is counter to the method taught herein.